1. Field of the Invention
The present invention relates to an improved tracking apparatus employing a four-horn monopulse system.
2. Description of the Related Art
Conventionally, there is known a tracking apparatus employing a four-horn monopulse system as a means for finding a direction by making use of a radio wave. FIG. 1 schematically shows the structure of the tracking apparatus of the four-horn monopulse system, wherein 1, 2, 3 and 4 designate electromagnetic horns disposed symmetrically to each other in an azimuthal angle direction and elevation angle direction in a 2.times.2 matrix state. The electromagnetic horns 1 and 2 and electromagnetic horns 3 and 4 are disposed in the azimuthal angle direction, respectively; and the electromagnetic horns 1 and 3 and electromagnetic horns 2 and 4 are disposed in the elevation angle direction, respectively. Designated at 5, 6, 7 and 8 are hybrid circuit. The hybrid circuit 5 creates a sum signal (A+B) and difference signal (A-B) of the respective signals A and B received by the electromagnetic horns 1 and 2; and the hybrid circuit 6 creates a sum signal (C+D) and difference signal (C-D) of the respective signals C and D received by the electromagnetic horns 3 and 4. Further, the hybrid circuit 7 is supplied . with the sum signal (A+B) created by the hybrid circuit 5 and the sum signal (C+D) created by the hybrid circuit 6 and creates a sum signal [(A+B)+(C+D)] thereof and a difference signal [(A+B)-(C+D)] thereof; whereas the hybrid circuit 8 is supplied with the difference signal (A-B) created by the hybrid circuit 5 and the difference signal (C-D) created by the hybrid circuit 6 and creates a sum signal [(A-B)+(C-D)=(A+C)-(B+D)] thereof and a difference signal [(A-B)-(C-D)] thereof.
According to the thus arranged 4-horn monopulse tracking apparatus, the difference signal [(A+B)-(C+D)] created by the hybrid circuit 7, i.e., the difference signal [(A+B)-(C+D)] obtained from the sum signal (A+B) of the electromagnetic horns 1 and 2 and the sum signal (C+D) of the electromagnetic horns 3 and 4 is used as an elevation angle error signal; and the sum signal [(A-B)+(C-D)=(A+C)-(B+D)] created by the hybrid circuit 8, i.e., the difference signal [(A+C) -(B+D)] obtained from the sum signal (A+C) of the electromagnetic horns 1 and 3 and the sum signal (B+D) of the electromagnetic horns 2 and 4 is used as a azimuthal angle error signal. Thus, the two-dimensional position of a radio wave emitting object can be determined by using these error signals. More specifically, although two or more kinds of information with respect to directions different to each other is generally needed to determine the two-dimensional position of an object, the aforesaid four-horn monopulse tracking apparatus can determined the two-dimensional position of the radio wave emitting object because it is provided with the two signals of the azimuthal angle error signal and elevation angle error signal.
Incidentally, since the conventional monopulse tracking apparatus is provided with only the two kinds of information as described above, when information cannot be obtained in one of the two systems for the azimuthal angle and elevation angle due to the failure of the apparatus, or the like, the tracking apparatus cannot determine a position. To prevent the occurrence of such a situation, both the azimuthal angle system and elevation angle system must be arranged to a redundant structure. Since a usual structure needs four hybrid circuits, eight hybrid circuits which are twice those of the usual structure are needed to provide the redundant structure, and thus the conventional monopulse tracking apparatus is disadvantageous in that the structure thereof is very complex.